1. Field of the Invention
This invention relates to an improved coal conversion process and more particularly relates to a process for devolatilizing and gasifying coal.
2. Description of the Prior Art
A variety of methods are known in the art for converting solid coal into normally liquid and normally gaseous hydrocarbons; a number of widely varying processes for carbonization and hydrocarbonization of coal are now available. Temperatures of individual reaction stages vary from 800.degree. to 3000.degree. F. and pressures range from near atmospheric to over 1000 psi. Contacting modes include fixed bed, fluidized bed, and dispersed phase. In any particular reaction stage, the gas-solids contacting mode may be either concurrent, counter-current, or backmixed, with perhaps another mode of contact from stage to stage. The number of reaction stages may vary from a simple, single contact through multiple-bed configurations, to essentially infinite stages in the continuously-variable temperature, fixed-bed gasifiers.
Low-temperature carbonization of coal with by-product recovery of liquids and gases has received growing attention in recent years. In the low temperature carbonization of coal (e.g., from about 750.degree. to 1800.degree. F.), the quantity of gaseous products is small and that of liquid products is relatively large; in the high-temperature carbonization of coal (e.g., at temperatures greater than about 1650.degree. F.), the yield of gaseous products is larger than the yield of liquid products, the production of char being relatively low. During the 1950's, efforts were directed toward production of char for fuel and little attention was given to optimization of tar and oil yields. More recent investigations have led to the concept of hydrocarbonization, wherein the production of coal liquids is emphasized.
Hydrocarbonization processes are similar to low-temperature carbonization in that fluidized or entrained beds are used. However, pressurized hydrogen is used as the fluidizing medium instead of air. Hydrogen tends to increase the yields of liquids, particularly as the hydrogen partial pressure is increased. But the hydrocarbonization processes should be distinguished from high-pressure hydrogenation processes. Although the liquid yields for hydrocarbonization are somewhat lower than for hydrogenation, the overall yields of gas plus liquids are comparable. Moreover, the difficult "pasting" oil recycle and heavy oil letdown problems encountered in hydrogenation processes are avoided with hydrocarbonization.
A process being developed for the very rapid pyrolysis of coal is the Garrett Pyrolysis process described by Sass, "The Garrett Research and Development Co. Process for the Conversion of Coal into Liquid Fuels," presented at the 65th annual A.I.Ch.E. meeting, New York, New York, Nov. 29, 1972. The process is characterized as an entrained-bed, low-temperature carbonization with a short residence time that produces a high yield of liquid fuels. Feed coal is dried, pulverized, and conveyed with recycle product gas to the pyrolysis reactor. In the reactor, the coal is heated to 579.degree. C. (about 1075.degree. F.) by recycled char as the solids are conveyed through the pyrolysis zone at high velocity. The stream then passes through cyclones and a portion of the separated char is cooled and sent to storage. The remaining solids are reheated by partial combustion with air to a temperature of 649.degree. to 871.degree. C. (1200.degree. to 1600.degree. F.) in the char heater which acts as an entrained-bed reactor. Then the reheated solids are reconveyed to the pyrolysis reactor. Gases from the reactor are cooled and scrubbed to collect the product tar. Make-gas may be purified and converted to a marketable pipeline gas or used within the plant as a source of hydrogen. The char is useful as a fuel. The attractiveness of the Garrett process is said to be the high liquid yields obtained at low pressures by the rapid conductive heating of the small coal particles and by the short exposures to the high-temperature reactor, which prevent thermal decomposition of the tars in the reactor. At hydrogen partial pressures near one atmosphere and temperatures of 579.degree. C. to 590.degree. C. liquid yields up to 33 percent were reportedly achieved in the Garrett process.
The Garrett process is essentially the hydrocarbonization process analog of the "transfer line" carbonization process described in U.S. Pat. No. 2,608,526. In the latter process, carbonaceous feed is contacted with a finely-divided, highly turbulent mass of solids preheated to 1000.degree. to 1600.degree. F. in an elongated solids transfer line of relatively small cross section for a time sufficient to carbonize the feed but insufficient to permit appreciable cracking of volatile products. The solids residence time is within the range from about 0.5 to 15 seconds. The heat-carrying solids may be hot coke or inert solids such as sand, clay, etc. (column 3, line 33). The feed is quickly heated in the transfer line carbonizer to a temperature of 850.degree. to 1200.degree. F. and volatile products and char are withdrawn together with the propelling gas (steam, CO.sub.2, product gas, or the like).
The volatile products withdrawn from the carbonization process tend to be nearly as complex and refractory as the initial substance (the coal) from which it was distilled. Accordingly, U.S. Pat. No. 3,244,615 suggests the immediate catalytic hydrogenation of coal volatiles produced by destructive distillation of coal (in the presence or absence of hydrogen) over a catalyst comprising Co-Mo on alumina. The catalytic hydrogenation is accomplished by vapor phase surface catalysis of the volatile matter immediately following destructive distillation of the coal in a catalyst chamber separate from the distillation zone. A patent related to U.S. Pat. Nos. 3,244,615 is 3,247,092. Also, see U.S. Pat. No. 3,736,233 which teaches a transfer line coking process employing an iron oxide sulfur acceptor.
Methods for producing gases from coal or coal char are also well known in the art. Currently, there are only three coal gasification processes in commercial operation: (1) Lurgi fixed bed gasifier, (2) Koppers-Totzek entrained gasifier and (3) Winkler fluid bed process. Numerous other gasification schemes such as Bi-Gas, Hygas, etc. have been proposed and are at various stages of development. Only Lurgi recovers the hydrogen-rich volatile fractions from coal as by-products, but these co-products (tars, phenols, naphtha, etc.) are not considered valuable as they cause process complications and downstream handling difficulties. Further, it would appear that for coal gasification schemes in general, large quantities of potentially valuable organic compounds are unnecessarily degraded to gas and char.
Gasification may be envisioned as a sequence of three distinctly different steps:
1. Pyrolytic decomposition of the coal and production of volatile matter (Devolatilization). PA1 2. Interaction of volatile species under reducing conditions to produce gases and char (Reductive Gasification--Recoking). PA1 3. Interaction of coke and char under oxidizing conditions to produce additional gases (Oxidative Gasification). PA1 (1) reacting a solids fraction comprising char and catalyst with an oxygen-containing gas and steam in a regenerator/gasifier to produce synthesis gas and hot, regenerated catalyst which is recycled to the catalytic devolatilization zone; PA1 (2) reacting a first solids fraction comprising char with an oxygen-containing gas and steam in a gasifier to produce synthesis gas and separately reacting a second solids fraction comprising catalyst with an oxygen-containing gas to regenerate the catalyst; and PA1 (3) combusting (either partially or completely) a solids fraction selected from the group comprising: PA1 to generate steam and/or to produce a CO-rich stream which may subsequently be converted to synthesis gas by the water-gas shift reaction.
Presently all three of these steps are conducted in one reactor. See, for example, McCaleb, T. L., and Chen, C. L., "Coal Processing: Low BTU Gas is an Industrial Fuel," Chemical Engineering Progress, Volume 73, No. 6, pages 82-88, American Institute of Chemical Engineers (June, 1977).
In conventional pyrolytic processes for producing volatile matter from coal (discussed above), the yields of potential fuels or chemicals are rather low although the recent development of flash pyrolysis processes wherein the coal is rapidly heated results in higher yields of volatile matter. Typical values are 5-25 percent tar, 2-5 percent H.sub.2 and hydrocarbon gases, and 0-10 percent carbon oxides. The tar constituents can be separated into various chemicals but such separations are very expensive. Generally, tars are burned to generate the heat for the process which has as its major objective to produce coke. Because the yields of these materials are low in conventional pyrolytic processes, there is relatively little incentive to develop a large-scale conversion of coal for their production.
Gasification as presently practiced involves during the initial stages a pyrolysis very similar to that described above with the resultant char being gasified through partial oxidation of the solid. The initially volatilized materials are also converted to gases in a homogeneous gas phase reaction. This gas phase reaction, although faster than gas-solid reactions, is less efficient as the volatiles are released near the entry of the coal and often near the exit of the product gases. The yield of tar of this type is a detriment to the overall process for two reasons: (1) it is admixed with large quantities of water and thus phenolic materials occur in highly diluted aqueous phases and (2) it can cause plugging or corrosion of reactors and lines. The polymerization (recoking of tar) is accentuated by oxygen. Thus coking of tar occurs in the gasifier. This coke is of lower surface area and therefore of lower reactivity than residual coke from the devolatilized coal.
This recoking of tar has been recently shown to be much more important than previously suspected. D. B. Anthony (AIChE Journal 22 622 (1976)) has reviewed extensively the problems of coal devolatilization and shown quite conclusively that by rapidly heating coal and quenching the products, much higher yields of volatile products are possible. The major cause for this increased yield is due to increased tar formation.
The exact chemical nature of the tar is not known. However, it is known that the selectivity of the tar-char reaction can be dramatically affected by the presence of solid materials. In inert atmospheres it is believed that solid surfaces promote the recoking of the tar on the solid surfaces. However, in the presence of reaction gases, solid surfaces can promote favorable gasification reactions.
Hydrogen is one reaction gas that has been studied extensively. Rapid heating of coal in hydrogen atmospheres can result in well over 50 percent volatile yields. In the presence of solid catalysts, volatile yields as high as 90 percent have been reported (the majority of which occurs in less than 1 second). The primary products of gasification in the presence of H.sub.2 is CH.sub.4. However, other heavier products are also accentuated. It is believed that the initial step in this overall scheme is the devolatilization of the coal to small reactive fragments. These can either recombine or react with other species such as H.sub.2.
After the relatively fast reactions of devolatilization, reductive gasification and recoking, the residual solid material (coke) is then gasified under oxidative conditions. This generally is carried out in the lower regions of present gasifiers by direct combustion with oxygen (partial and/or complete). An intermediate gasification of char also occurs due to the interaction of char with hot CO, CO.sub.2, and steam. This intermediate stage is the predominant reaction which leads to the production of CO and H.sub.2.
An object of the present invention is to provide a coal devolatilizing and conversion process wherein more desirable volatile product fractions of the coal are efficiently recovered.
Another object of this invention is to provide a more desirable environment for char formation during coal devolatilization which will provide a coal char highly desirable for subsequent use including gasification. A further object of this invention is to provide a coal devolatilizing process which improves the direct production of premium liquid hydrocarbon products. A further object of this invention is to furnish a unique method for ash removal.